(215d) A Tunable Voltage-Gated Nanochannel for Electrokinetic Sample Preconcentration

In electrokinetic phenomena, the distinction between "nano"-scale and "micro"-scale is an important one, since the former refers specifically to situations involving the overlap of nano-meter sized electrical double layers surrounding adjacent charged surfaces. Indeed, double layer overlap in nanochannels has been utilized by various researchers as a means of affecting ion-selective transport and sample preconcentration of bio-molecules. In these cases, however, the ion selectivity is determined by the native zeta potential of the channel and ionic concentration of the electrolytic solution, quantities that are difficult to vary independently. In this talk, we present theoretical and experimental investigations on a novel voltage-gated nanochannel, wherein metal electrodes are deposited on portions of the top and bottom surfaces of a fused silica nanochannel. An electric field or pressure difference applied axially drives ionic transport along the nanochannel, and by actively controlling the voltage of the gate electrodes we are able to tune the local zeta potential --- and hence ion selectivity --- of the nanochannel. A theoretical analysis is presented to elucidate the degree of ion enrichment (preconcentration) and depletion at opposite sides of the gate, as a function of gate voltage, driving voltage, electrode length (relative to the nanochannel), and ionic concentrations. Experimentally, we visualize and quantify the ionic enrichment and depletion zones using fluorescent tracer molecules and a robust epifluorescent experimental setup.